Multicomponent systems based on polypyrrole

ABSTRACT

Compositions based on polypyrrole are described having incorporated therein a polyether, such as polytetrahydrofuran. The polyether is present in an amount of at least about 10 weight percent of the polypyrrole present. The compositions exhibit improved processing properties over these of polypyrrole. A process for producing the compositions is also described in which a pyrrole compound is electrochemically polymerized in the presence of a dissolved polyether.

FIELD OF THE INVENTION

This invention is in the field of polymer chemistry. More specifically,this invention relates to polypyrrole, an aromatic polyheterocycle.

BACKGROUND OF THE INVENTION

Conducting polymers have been the subject of intense research activityfor the past decade. Currently, much work is devoted to the synthesis ofconducting polymers for use in a variety of applications. Polyacetylene,the prototype conducting polymer, has been successfully demonstrated tobe useful in constructing p-n hetero-junctions, Schottky barrier diodes,liquid junction photoelectrochemical solar cells, and, more recently, asthe active electrode in polymeric batteries.

The improved electrochemical synthesis of polypyrrole has led to its useas coating for the protection of n-type semiconductors againstphotocorrosion in photoelectrochemical cells. Research studies haveshown that pyrrole and thiophene, five-membered heterocyclic aromaticring compounds, undergo simultaneous oxidation and polymerization.Conducting polyheterocycles, such as polypyrrole and polythiophene, havedemonstrated dramatic improvement in oxidative stability over otherconducting polymers. However, polyheterocycles, like other knownconducting polymers, are hampered by inferior mechanical properties.

Polypyrrole and polythiophene can be synthesized by electrochemicaltechniques on the surface of an electrode. Electrochemically synthesizedpolythiophene films are extremely brittle, making it difficult to removethe films from electrode surfaces without fragmentation.Electrochemically synthesized films of polypyrrole are less brittle thanpolythiophene films. Heretofore, electrochemically synthesizedpolypyrrole has exhibited poor processibility, i.e., has been difficultto process.

SUMMARY OF THE INVENTION

The present invention is directed to improving the processibility ofelectrochemically synthesized polypyrrole.

According to one aspect of the present invention a composition isprovided which is a multicomponent system comprising a solid polypyrrolehaving incorporated therein a polyether, the polyether being present inan amount of at least about 10 weight percent of the polypyrrole presentin the multicomponent system.

In another aspect of the present invention there is provided a methodfor preparing multicomponent systems comprising a polypyrrole and apolyether. The method involves electrochemically polymerizing a pyrrolecompound in the presence of a dissolved polyether. More specifically,the method involves passing a current across an electrochemical cellcontaining a solution of an electrolyte, a polyether, and a pyrrolecompound in a solvent in order to form a multicomponent system film onthe anode of the cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an electrochemical cell suitablefor use in the method of the present invention.

FIG. 2 is a graphical comparison of the conductivity of a multicomponentsystem comprising polythiophene and polytetrahydrofuran to that ofpolythiophene, as measured at 110° C. in laboratory air.

For a better understanding of the present invention, together with otherand further objects, advantages and capabilities thereof, reference ismade to the following disclosure and appended claims in connection withthe above described drawings.

DETAILED DESCRIPTION OF THE INVENTION

The multicomponent system of the present invention is a compositionwhich comprises a solid polypyrrole and a polyether. The polyether ispresent in an amount of at least about 10 weight percent of thepolypyrrole present. Preferably, the polyether is present in an amountfrom about 10 to about 30 weight percent of the polypyrrole present. Themulticomponent system of the present invention has improvedprocessibility and mechanical integrity over polypyrrole. An additionaladvantage associated with the multicomponent system of the presentinvention over polypyrrole is that the d.c. electrical conductivity ofthe multicomponent system is increased by annealing the multicomponentmaterial in air. A further advantage associated with the multicomponentsystem of the present invention is its processibility, i.e., the systemcan be molded or otherwise processed using a combination of heat andpressure.

The properties of a multicomponent system having a polyether contentless than about 10 weight percent of the polypyrrole present exhibitlittle or no improvement over those of polypyrrole. The incorporation ofa polyether content above thirty weight percent of the polypyrrolepresent produces a material which has improved processibility andmechanical integrity over polypyrrole, but which is more insulating thanpolypyrrole.

The method of the present invention produces a multicomponent systemcomprising a polypyrrole and a polyether. This method includes passing acurrent across an electrochemical cell which contains a solution of anelectrolyte, a polyether, and a pyrrole compound in a solvent. Theapplication of the current across the solution results in the formationof a film, or layer, of the polypyrrole/polyether multicomponent systemon the anode. The polypyrrole/polyether film produced in accordance withthe method of the present invention is a "pliant", i.e., the film can beremoved from the electrode surface on which it is grown and can behandled and processed without breaking or fragmenting.

For a pyrrole compound to be suitable for use in the present method, itmust be capable of being electrochemically polymerized. Examples ofpolymerizable pyrrole compounds include: ##STR1## The concentration ofthe pyrrole compound in the solution of this method should be at least0.01 mole/liter (M). Preferably the pyrrole compound concentration isfrom about 0.01 M to about 1.0 M.

The choice of solvent employed to form the solution of this invention isnot critical. Any suitable solvent may be used. A suitable solvent is asolvent in which the pyrrole compound is soluble and in which thepolymerized pyrrole compound is insoluble. Additionally, the solventmust be electrochemically inert at the potential at which the pyrrolecompound is oxidized. Examples of typical solvents include benzonitrile,tetrahydrofuran, acetonitrile, nitrobenzene, and nitromethane.

Selection of an electrolyte and a polyether suitable for use in thepresent method is dependent upon the particular solvent selected. Anyelectrolyte which is soluble in the solvent being used and which iselectrochemically inert at the potential at which the pyrrole compoundis oxidized is a suitable electrolyte for use in the present method. Forexample, lithium perchlorate (LiClO₄) and tetrabutyl ammoniumtetrafluoroborate ((C₄ H₉)₄ NBF₄) are suitable electrolytes for use witha tetrahydrofuran or acetonitrile solvent.

A polyether is suitable for use in the method of the present inventionprovided the polyether is soluble in the solvent of the method. Forexample, polytetrahydrofuran is a suitable polyether for use with atetrahydrofuran solvent. Polyoxyethylene, polyoxypropylene, andpoly(oxy-1,3-phenylene) are other examples of polyethers which aresoluble in acetonitrile and/or tetrahydrofuran. Accordingly, thesepolyethers are typical polyethers for use in the present method.Preferably the polyether is polytetra-hydrofuran.

The films prepared by the process of this invention can be grown tovarious thicknesses on the working electrode of the cell with differentcurrent rates. The film thickness can be also controlled by varying theperiod of time during which a current is applied across the cell. As thethickness of the film increases, the surface of the film becomesnodulose.

The polymerization current density should be at least 0.1 mA/cm².Preferably, the polymerization current density used with respect to theanode is from 0.1 mA/cm² to about 1 mA/cm². Most preferably the currentdensity is about 0.8 mA/cm².

The process for preparing a polypyrrole/polyether multicomponent systemmay be carried out in an electrochemical cell such as theelectrochemical cell of FIG. 1. A working electrode 10, the anode, isplaced in a first compartment 2. A counter electrode 14, the cathode, isplaced in a second compartment 4. A reference electrode 18 is placed ina third compartment 6. A current is applied to the working electrode,and passes through the solution contained in the cell for a period oftime sufficient to form a film of predetermined thickness on the surfaceof the working electrode, which is the anode of the cell. The workingelectrode is separated from the counter electrode by a medium porosityfrit 22. The reference electrode makes contact with the solution via alugin capillary 26.

Both potentiostatic and galvanostatic conditions can be used tosynthesize the multicomponent system. The polypyrrole/polyether can begrown on a platinum or gold working electrode as, well as on an SnO₂coated glass working electrode.

The films which are synthesized by the process of this invention areflexible, air stable, and have conductivities from 10⁻² to 1 (ohm-cm)⁻¹.

Neutral films can be produced by reversing the direction of current flowuntil the potential of the working electrode indicates the polymericfilm is no longer being reduced to the neutral state.

In a preferred embodiment of the method of this invention, the solutionof the process comprises a pyrrole compound, polytetrahydrofuran, and anelectrolyte in a solvent of tetrahydrofuran.

Most preferably, polytetrahydrofuran is electrochemically synthesized insitu, before the pyrrole compound is added to the cell, by applying acurrent across the electrochemical cell which contains a solutionconsisting essentially of tetrahydrofuran and an electrolyte.

The following example will more completely illustrate the practice ofthis invention. It will, be readily understood by those skilled in theart that the example should not be construed as limiting the scope ofthis invention in any way.

While the following is directed to a multicomponent system based onpolythiophene, a multicomponent system based on polypyrrole will behavein an analagous manner.

The cell of FIG. 1 was used for the electrochemical synthesis of amulti-component system incorporating polythiophene andpolytetrahydrofuran. 50 ml of a 1M solution of LiClO₄ in tetrahydrofuran(THF) was added to the cell. A platinum (Pt) working electrode 10 wasplaced in the first compartment 2 and a nickel (Ni) counter electrode 14was placed in the second compartment 4. A reference electrode 18 wasplaced in the third compartment 6. A constant current of 0.8 mA/cm² wasapplied to the working electrode and current was passed for a durationof 4.8 coulombs/cm². During this time, polymerization of tetrahydrofuranoccurred. The formation of polytetrahydrofuran causes the solution tobecome viscous as some of the polytetrahydrofuran precipitates out ofsolution. After the passage of 4.8 coulombs/cm² across the cell to formpolytetrahydrofuran, 0.1 gram of 2,2'-bithiophene was added to the firstcompartment 2 which contained approximately 25 ml of the solutioncontaining LiClO₄ and polytetrahydrofuran. An additional 4.8-8coulombs/cm² was passed through the solution containing the2,2'-bithiophene at a current density of 0.8 mA/cm². While the currentwas applied across the cell, a film grew on the working electrodesurface. The thickness of the film was controlled by the coulombs/cm²which was passed. The film was removed from the electrode surface andwashed in tetrahydrofuran to remove unreacted 2,2'-bithiophene, LiClO₄,and polytetrahydrofuran from the surface of the film. The film waspliant, which permitted the film to be removed from the electrodewithout the film fragmenting. The film produced had a thickness of ˜100microns.

The polythiophene/polytetrahydrofuran film was analyzed via scanningelectron microscopy (SEM). The film was subsequently extracted withtetrahydrofuran. After extraction, the film was again analyzed under thescanning electron microscope. Infrared spectroscopy of the materialisolated during the extraction revealed that the film containedpolytetrahydrofuran. The SEM photomicrographs showed that thepolytetrahydrofuran present in film interconnects nodules ofpolythiophene. The film contained approximately 20% polytetrahydrofuran,as confirmed by weight loss after extraction.

The polytetrahydrofuran content of the film can be manipulated to anextent by varying the conditions. For example, the polytetrahydrofurancontent of a polythiophene/polytetrahydrofuran multicomponent system canbe increased by polymerizing THF for a longer period of time andpolymerizing thiophene for a shorter period.

The tensile strength of the polythiophene/polytetrahydrofuranmulticomponent system of the example was 1000 psi (70.30 kg/cm². Theelongation of the multicomponent system film at the break (1_(f) /1_(o))was 1.03. A comparison of the tensile strength of thepolythiophene/polytetrahydrofuran film, prepared in accordance with thisinvention, with the tensile strength of a film consisting only ofpolythiophene could not be made because a sample of electrochemicallysynthesized polythiophene film of suitable size for measuring tensilestrength could not be removed from the electrode without fragmenting.The mechanical integrity of polythiophene is shown to be improved viathe method of the present invention by the fact that the multicomponentfilm can be manipulated without breaking or fragmenting.

The d.c. electrical conductivity of thepolythiophene/polytetrahydrofuran film produced by the present method isfrom 10⁻² to 1.0 (ohm-cm)⁻¹, depending upon polymerization conditions.The conductivity is increased by annealing the film in air. FIG. 2presents accelerated stability life data for polythiophene and for apolythiophene/polytetrahydrofuran (polyTHF) multi-component system. Morespecifically, FIG. 2 compares the conductivity of a polythiophene sampleat 110° Celsius as a function of time to the conductivity of apolythiophene/polyTHF multicomponent system sample at 110° C. as afunction of time. In FIG. 2, conductivity is represented as log σ_(T)/σ₀, where σ_(T) is the conductivity at time=T and σ₀ is theconductivity at time=0. For both polythiophene and thepolythiophene/polyTHF multicomponent system σ₀ was measured to be 0.02(ohm-cm)⁻¹. FIG. 2 shows that the conductivity of thepolythiophene/polyTHF multicomponent system remained constant after ashort initial thermal activation process while the conductivity of thepolythiophene decreased after a short initial thermal activationprocess. After cooling to room temperature, the conductivity of themulticomponent system was about 10% higher than the conductivity priorto the high temperature life test. After cooling to room temperature,the conductivity of polythiophene was about 20% less than prior toheating.

The incorporation of polytetrahydrofuran into films of polythiophene, asdescribed above, enables the multicomponent system to be processed aftersynthesis, i.e., the multicomponent system in accordance with thepresent invention can be molded into desirable shapes using lowtemperatures and pressures. For example, washers of the multicomponentwere fabricated using a teflon mold. The washers were fabricated bycutting the polythiophene/polytetrahydrofuran multicomponent materialinto pieces, loading it into the mold and heating the mold to 76 degreescelsius under a constant pressure of about 20 psi (1.4 kg/cm²).

Wide band absorption testing conducted on thepolythiophene/polytetrahydrofuran multicomponent system of thisinvention has shown the composition to be a radar absorbing material(RAM). Measurements of the electric permitivity of thepolythiophene/polytetrahydrofuran multicomponent system as a function offrequency in the 2-18 GHz range showed the multicomponent system to beabsorbing in the 2-18 GHz range. Absorption beyond this range was notmeasured, but is clearly anticipated.

The material of the present invention exhibits thermal and oxidativestability is processible, and exhibits a higher degree of mechanicalintegrity than polypyrrole.

What is claimed is:
 1. A composition comprising a solid polypyrrolehaving incorporated therein polyether, the polyether being present in anamount of at least about 10 weight percent of the polypyrrole present.2. A composition in accordance with claim 1 wherein the polyether ispresent in an amount from 10 weight percent to about 30 weight percentof the polypyrrole present.
 3. A composition in accordance with claim 1wherein the polypyrrole is prepared by electrochemical polymerization ofa pyrrole compound selected from the group consisting of: ##STR2##
 4. Acomposition in accordance with claim 3 wherein the pyrrole compound ispyrrole.
 5. A composition in accordance with claim 3 wherein thepolyether is selected from the group consisting of polytetrahydrofuran,polyoxyethylene, polyoxypropylene, and poly(oxy-1,3-phenylene).
 6. Acomposition in accordance with claim 5 wherein the polyether ispolytetrahydrofuran.
 7. A method for preparing a multicomponent systemcomprising a polypyrrole and a polyether, said method comprising:passinga current across an electrochemical cell containing a solutioncomprising a pyrrole compound, an electrolyte, and a polyether in asolvent to form a multicomponent system film of predetermined thicknesson the anode of the cell, wherein: said pyrrole compound iselectrochemically polymerizable; said solvent is selected such that saidpyrrole compound is soluble therein and the polypyrrole is insolubletherein, said electrolyte is soluble in said solvent; and said polyetheris soluble in said solvent.
 8. A method in accordance with claim 7wherein the density of the current flow to the anode is at least 0.1mA/cm².
 9. A method in accordance with claim 7 wherein the pyrrolecompound is pyrrole and the polyether is polytetrahydrofuran.
 10. Amethod for preparing a multicomponent system comprising a polypyrroleand a polyether, said method comprising:passing a current across anelectrochemical cell containing a solution of an electrolyte intetrahydrofuran to form a polytetrahydrofuran precipitate in thesolution; adding an electrochemically polymerizable pyrrole compound tothe solution contained in said electrochemical cell; and passing acurrent across said electrochemical cell containing the solutioncomprising said pyrrole compound, said polytetrahydrofuran precipitate,and said electrolyte in tetrahydrofuran to form a multicomponent systemfilm of predetermined thickness on the anode of the cell.